The present invention relates to a device for detecting the presence of coherent radiation or the coherent absence of radiation in the presence of non-coherent background radiation. More particularly, the present invention relates to the use of an imaging optical radiometer to make such detections and determine the direction and wavelength of such radiation or such lack of radiation.
Imaging optical radiometers, constructed in accordance with the concept of this invention are adapted, among other possible uses for detecting and determining the wavelength of coherent radiation or the coherent absence of radiation. In addition, it can be used to determine the direction of arrival of the source of coherent radiation or coherent absence of radiation and indicate such position in a display of the field of view.
Such a device can find application in specific gas cloud detection, oil and mineral exploration and detection through Fraunhofer line discrimination techniques, and intelligence surveillance.
Conventional laser receivers use a narrow-band optical filter or diffraction gratings in combination with a photodetector, bandpass amplifier and thresholded peak detector to detect the presence of coherent radiation. This approach has two disadvantages: one, the laser wavelength must be known and two, the video bandwidth required to pass nanosecond pulses also passes a lot of detector and/or background photon noise. The coherent radiometer approach has a broad spectral response and a noise integration time limited only by the available observation time.
The prior art is evidenced by U.S. Pat. Nos. 3,824,018 to R. Crane Jr. and U.S. Pat. No. 4,309,108 to E. Seibert, both of which are assigned to the same assignee as the present application. The aforementioned patents disclose the use of Fabry-Perot etalon interferometers.
While the prior art devices detect presence, wavelength and direction of arrival of coherent radiation from a single source, our contribution is to do so for all coherent sources within a scene, resulting in an imaging coherent radiometer with longer integration times for sensitivity enhancement, to also do so for coherent absence of radiation, and for other advantages, as will become apparent as the description proceeds.